I am not sure that I can answer your question definitively. There are several reasons for this.
1) OP churches vary widely in what they offer in terms of Sunday School classes. There are a number of possible scenarios:
a) Some churches, like my own, have classes for younger teenagers, but when the teens are juniors or seniors or college age, they may be blended in with an adult class which is, almost without exception, taught by a man. In this case the situation does not arise.
b) In some OP congregations there may be a special class for teenagers team-taught by, say, a husband and wife, in which case the husband provides the leadership.
c) In some churches there may be separate male and female classes for teens in which case it would be normal for the boys' class to be taught by a man.
d) In some churches there is only one "family-oriented" class and there is no age or peer division and the class is taught by the pastor or an elder.
So, there are a number of ways in which the question you pose would not arise.
2) Each OP congregation is under the authority of its own session and the session is responsible for ordering things in the way they see is most edifying to the people of the congregation. But, at the same time, each session is bound by the Word of God and the standards of the OPC which both set forth that leadership in the church is to be male (1 Tim. 2:12). A session could construe this to mean that as long as the elders (male) diligently oversee the whole of the life of the church, and people, both men and women, are submissive to that leadership, the elders could delegate responsibility for doing certain things, in this case teaching a class, to whomever they deem best suited to do that, be they male or female. This is what I mean: routinely elders will delegate the leading of a women's Bible to a woman but the final responsibility and authority rests or remains with men (the session). So, presumably, a session could permit or request a woman a teach of Sunday School class which might include an eighteen year old male but recognize that they themselves are the ones with the authority to approve the teacher or the material, etc. I don't know of any OP congregations where the elders do it that way but since the elders are the spiritual undershepherds of the flock and everything that happens in it (cf. Acts 20:28, Heb. 13:17), they could do this.
In summary, the OPC believes 1 Timothy 2:12 is truth, and how that is implemented in each congregation rests with the authority of the local session to operate within the boundary of the Word of God and our Book of Church Order.
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